Evidence for a sensitising pigment in fly photoreceptors

Drosophila photoreceptor systems converge in arousal neurons and confer light responsive robustness

Lateral ventral neurons (LNvs) in the fly circadian neural circuit mediate behaviors other than clock resetting, including light-activated acute arousal. Converging sensory inputs often confer functional redundancy. The LNvs have three distinct light input pathways: (1) cell autonomously expressed cryptochrome (CRY), (2) rhodopsin 7 (Rh7), and (3) synaptic inputs from the eyes and other external photoreceptors that express opsins and CRY. We explored the relative photoelectrical and behavioral input contributions of these three photoreceptor systems to determine their functional impact in flies. Patch-clamp electrophysiology measuring light evoked firing frequency (FF) was performed on large LNvs (l-LNvs) in response to UV (365 nm), violet (405 nm), blue (450 nm), or red (635 nm) LED light stimulation, testing controls versus mutants that lack photoreceptor inputs gl60j, cry-null, rh7-null, and double mutant gl60j-cry-null flies. For UV, violet, and blue short wavelength light inputs, all photoreceptor mutants show significantly attenuated action potential FF responses measured in the l-LNv. In contrast, red light FF responses are only significantly attenuated in double mutant gl60j-cry-null flies. We used a light-pulse arousal assay to compare behavioral responses to UV, violet, blue and red light of control and light input mutants, measuring the awakening arousal response of flies during subjective nighttime at two different intensities to capture potential threshold differences (10 and 400 μW/cm2). The light arousal behavioral results are similar to the electrophysiological results, showing significant attenuation of behavioral light responses for mutants compared to control. These results show that the different LNv convergent photoreceptor systems are integrated and together confer functional redundancy for light evoked behavioral arousal.

High diversity of arthropod colour vision: from genes to ecology

Colour vision allows animals to use the information contained in the spectrum of light to control important behavioural decisions such as selection of habitats, food or mates. Among arthropods, the largest animal phylum, we find completely colour-blind species as well as species with up to 40 different opsin genes or more than 10 spectral types of photoreceptors, we find a large diversity of optical methods shaping spectral sensitivity, we find eyes with different colour vision systems looking into the dorsal and ventral hemisphere, and species in which males and females see the world in different colours. The behavioural use of colour vision shows an equally astonishing diversity. Only the neural mechanisms underlying this sensory ability seems surprisingly conserved-not only within the phylum, but even between arthropods and the other well-studied phylum, chordates. The papers in this special issue allow a glimpse into the colourful world of arthropod colour vision, and besides giving an overview this introduction highlights how much more research is needed to fill in the many missing pieces of this large puzzle. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

From Photons to Behaviors: Neural Implementations of Visual Behaviors in Drosophila

Neural implementations of visual behaviors in Drosophila have been dissected intensively in the past couple of decades. The availability of premiere genetic toolkits, behavioral assays in tethered or freely moving conditions, and advances in connectomics have permitted the understanding of the physiological and anatomical details of the nervous system underlying complex visual behaviors. In this review, we describe recent advances on how various features of a visual scene are detected by the Drosophila visual system and how the neural circuits process these signals and elicit an appropriate behavioral response. Special emphasis was laid on the neural circuits that detect visual features such as brightness, color, local motion, optic flow, and translating or approaching visual objects, which would be important for behaviors such as phototaxis, optomotor response, attraction (or aversion) to moving objects, navigation, and visual learning. This review offers an integrative framework for how the fly brain detects visual features and orchestrates an appropriate behavioral response.

Mechanisms of vitamin A metabolism and deficiency in the mammalian and fly visual system

Vitamin A deficiency can cause human pathologies that range from blindness to embryonic malformations. This diversity is due to the lack of two major vitamin A metabolites with very different functions: the chromophore 11-cis-retinal (vitamin A aldehyde) is a critical component of the visual pigment that mediates phototransduction, while the signaling molecule all-trans-retinoic acid regulates the development of various tissues and is required for the function of the immune system. Since animals cannot synthesize vitamin A de novo, they must obtain it either as preformed vitamin A from animal products or as carotenoid precursors from plant sources. Due to its essential role in the visual system, acute vitamin A deprivation impairs photoreceptor function and causes night blindness (poor vision under dim light conditions), while chronic deprivation results in retinal dystrophies and photoreceptor cell death. Chronic vitamin A deficiency is the leading cause of preventable childhood blindness according to the World Health Organization. Due to the requirement of vitamin A for retinoic acid signaling in development and in the immune system, vitamin A deficiency also causes increased mortality in children and pregnant women in developing countries. Drosophila melanogaster is an excellent model to study the effects of vitamin A deprivation on the eye because vitamin A is not essential for Drosophila development and chronic deficiency does not cause lethality. Moreover, genetic screens in Drosophila have identified evolutionarily conserved factors that mediate the production of vitamin A and its cellular uptake. Here, we review our current knowledge about the role of vitamin A in the visual system of mammals and Drosophila melanogaster. We compare the molecular mechanisms that mediate the uptake of dietary vitamin A precursors and the metabolism of vitamin A, as well as the consequences of vitamin A deficiency for the structure and function of the eye.

Evolution of Insect Color Vision: From Spectral Sensitivity to Visual Ecology

Color vision is widespread among insects but varies among species, depending on the spectral sensitivities and interplay of the participating photoreceptors. The spectral sensitivity of a photoreceptor is principally determined by the absorption spectrum of the expressed visual pigment, but it can be modified by various optical and electrophysiological factors. For example, screening and filtering pigments, rhabdom waveguide properties, retinal structure, and neural processing all influence the perceived color signal. We review the diversity in compound eye structure, visual pigments, photoreceptor physiology, and visual ecology of insects. Based on an overview of the current information about the spectral sensitivities of insect photoreceptors, covering 221 species in 13 insect orders, we discuss the evolution of color vision and highlight present knowledge gaps and promising future research directions in the field.

The spectral sensitivity of Drosophila photoreceptors

Drosophila melanogaster has long been a popular model insect species, due in large part to the availability of genetic tools and is fast becoming the model for insect colour vision. Key to understanding colour reception in Drosophila is in-depth knowledge of spectral inputs and downstream neural processing. While recent studies have sparked renewed interest in colour processing in Drosophila, photoreceptor spectral sensitivity measurements have yet to be carried out in vivo. We have fully characterised the spectral input to the motion and colour vision pathways, and directly measured the effects of spectral modulating factors, screening pigment density and carotenoid-based ocular pigments. All receptor sensitivities had significant shifts in spectral sensitivity compared to previous measurements. Notably, the spectral range of the Rh6 visual pigment is substantially broadened and its peak sensitivity is shifted by 92 nm from 508 to 600 nm. We show that this deviation can be explained by transmission of long wavelengths through the red screening pigment and by the presence of the blue-absorbing filter in the R7y receptors. Further, we tested direct interactions between inner and outer photoreceptors using selective recovery of activity in photoreceptor pairs.

A novel setup for simultaneous two-photon functional imaging and precise spectral and spatial visual stimulation in Drosophila

Motion vision has been extensively characterised in Drosophila melanogaster, but substantially less is known about how flies process colour, or how spectral information affects other visual modalities. To accurately dissect the components of the early visual system responsible for processing colour, we developed a versatile visual stimulation setup to probe combined spatial, temporal and spectral response properties. Using flies expressing neural activity indicators, we tracked visual responses in the medulla, the second visual neuropil, to a projected colour stimulus. The introduction of custom bandpass optical filters enables simultaneous two-photon imaging and visual stimulation over a large range of wavelengths without compromising the temporal stimulation rate. With monochromator-produced light, any spectral bandwidth and centre wavelength from 390 to 730 nm can be selected to produce a narrow spectral hue. A specialised screen material scatters each band of light across the visible spectrum equally at all locations of the screen, thus enabling presentation of spatially structured stimuli. We show layer-specific shifts of spectral response properties in the medulla correlating with projection regions of photoreceptor terminals.

Photoresponses in the radiolar eyes of the fan worm A cromegalomma vesiculosum

Fan worms (Annelida: Sabellidae) possess compound eyes and other photoreceptors on their radiolar feeding tentacles. These eyes putatively serve as an alarm system that alerts the worm to encroaching threats, eliciting a rapid defensive retraction into their protective tube. The structure and independent evolutionary derivation of these radiolar eyes make them a fascinating target for exploring the emergence of new sensory systems and visually guided behaviours. However, little is known about their physiology and how this impacts their function. Here, we present electroretinogram recordings from the radiolar eyes of the fan worm Acromegalomma vesiculosum We examine their spectral sensitivity along with their dynamic range and temporal resolution. Our results show that they possess one class of photoreceptors with a single visual pigment peaking in the blue-green part of the spectrum around 510 nm, which matches the dominant wavelengths in their shallow coastal habitats. We found the eyes to have a rather high temporal resolution with a critical flicker fusion frequency around 35 Hz. The high temporal resolution of this response is ideally suited for detecting rapidly moving predators but also necessitates downstream signal processing to filter out caustic wave flicker. This study provides a fundamental understanding of how these eyes function. Furthermore, these findings emphasise a set of dynamic physiological principles that are well suited for governing a multi-eyed startle response in coastal aquatic habitats.

Melanopsin and the Intrinsically Photosensitive Retinal Ganglion Cells: Biophysics to Behavior

The mammalian visual system encodes information over a remarkable breadth of spatiotemporal scales and light intensities. This performance originates with its complement of photoreceptors: the classic rods and cones, as well as the intrinsically photosensitive retinal ganglion cells (ipRGCs). IpRGCs capture light with a G-protein-coupled receptor called melanopsin, depolarize like photoreceptors of invertebrates such as Drosophila, discharge electrical spikes, and innervate dozens of brain areas to influence physiology, behavior, perception, and mood. Several visual responses rely on melanopsin to be sustained and maximal. Some require ipRGCs to occur at all. IpRGCs fulfill their roles using mechanisms that include an unusual conformation of the melanopsin protein, an extraordinarily slow phototransduction cascade, divisions of labor even among cells of a morphological type, and unorthodox configurations of circuitry. The study of ipRGCs has yielded insight into general topics that include photoreceptor evolution, cellular diversity, and the steps from biophysical mechanisms to behavior.

Ultraviolet inflorescence cues enhance attractiveness of inflorescence odour to Culex pipiens mosquitoes

Inflorescence patterns of ultraviolet (UV) absorption and UV-reflection are attractive to many insect pollinators. To understand whether UV inflorescence cues affect the attraction of nectar-foraging mosquitoes, we worked with the common house mosquito, Culex pipiens and with two plant species exhibiting floral UV cues: the tansy, Tanacetum vulgare, and the common hawkweed Hieraciumm lachenalii. Electroretinograms revealed that Cx. pipiens eyes can sense UV wavelengths, with peak sensitivity at 335 nm. Behavioural bioassays divulged that UV inflorescence cues enhance the attractiveness of inflorescence odour. In the presence of natural floral odour, female Cx. pipiens were attracted to floral patterns of UV-absorption and UV-reflection but preferred uniformly UV-dark inflorescences. Moreover, Cx. pipiens females preferred UV-dark and black inflorescence models to UV-dark and yellow inflorescence models. With feathers and pelts of many avian and mammalian hosts also being UV-dark and dark-coloured, foraging Cx. pipiens females may respond to analogous visual cues when they seek nectar and vertebrate blood resources.

Artificial Rod and Cone Photoreceptors with Human-Like Spectral Sensitivities

Photosensitive materials contain biologically engineered elements and are constructed using delicate techniques, with special attention devoted to efficiency, stability, and biocompatibility. However, to date, no photosensitive material has been developed to replace damaged visual-systems to detect light and transmit the signal to a neuron in the human body. In the current study, artificial nanovesicle-based photosensitive materials are observed to possess the characteristics of photoreceptors similar to the human eye. The materials exhibit considerably effective spectral characteristics according to each pigment. Four photoreceptors originating from the human eye with color-distinguishability are produced in human embryonic kidney (HEK)-293 cells and partially purified in the form of nanovesicles. Under various wavelengths of visible light, electrochemical measurements are performed to analyze the physiological behavior and kinetics of the photoreceptors, with graphene, performing as an electrode, playing an important role in the lipid bilayer deposition and oxygen reduction processes. Four nanovesicles with different photoreceptors, namely, rhodopsin (Rho), short-, medium-, and longwave sensitive opsin 1 (1SW, 1MW, 1LW), show remarkable color-dependent characteristics, consistent with those of natural human retina. With four different light-emitting diodes for functional verification, the photoreceptors embedded in nanovesicles show remarkably specific color sensitivity. This study demonstrates the potential applications of light-activated platforms in biological optoelectronic industries.

Multichannel spectrometers in animals

Multispectral, hyperspectral, polarimetric, and other types of multichannel imaging spectrometers are coming into common use for a variety of applications, including remote sensing, material identification, forensics, and medical diagnosis. These instruments are often bulky and intolerant of field abuse, so designing compact, reliable, portable, and robust devices is a priority. In contrast to most engineering designs, animals have been building compact and robust multichannel imaging systems for millennia-their eyes. Biological sensors arise by evolution, of course, and are not designed 'for' a particular use; they exist because the creatures that were blessed with useful mutations were better able to survive and reproduce than their competitors. While this is an inefficient process for perfecting a sensor, it brings unexpected innovations and novel concepts into visual system design-concepts that may be useful in the inspiration of new engineered solutions to problematic challenges, like the ones mentioned above. Here, we review a diversity of multichannel visual systems from both vertebrate and invertebrate animals, considering the receptor molecules and cells, spectral sensitivity and its tuning, and some aspects of the higher-level processing systems used to shape spectral (and polarizational) channels in vision. The eyes of mantis shrimps are presented as potential models for biomimetic multichannel imaging systems. We end with a description of a bioinspired, newly developed multichannel spectral/polarimetric imaging system based on mantis shrimp vision that is highly adaptable to field application.

Retinal perception and ecological significance of color vision in insects

Color vision relies on the ability to discriminate different wavelengths and is often improved in insects that inhabit well-lit, spectrally rich environments. Although the Opsin proteins themselves are sensitive to specific wavelength ranges, other factors can alter and further restrict the sensitivity of photoreceptors to allow for finer color discrimination and thereby more informed decisions while interacting with the environment. The ability to discriminate colors differs between insects that exhibit different life styles, between female and male eyes of the same species, and between regions of the same eye, depending on the requirements of intraspecific communication and ecological demands.

Ectopic Expression of Mouse Melanopsin in Drosophila Photoreceptors Reveals Fast Response Kinetics and Persistent Dark Excitation

The intrinsically photosensitive M1 retinal ganglion cells (ipRGC) initiate non-image-forming light-dependent activities and express the melanopsin (OPN4) photopigment. Several features of ipRGC photosensitivity are characteristic of fly photoreceptors. However, the light response kinetics of ipRGC is much slower due to unknown reasons. Here we used transgenic Drosophila, in which the mouse OPN4 replaced the native Rh1 photopigment of Drosophila R1-6 photoreceptors, resulting in deformed rhabdomeric structure. Immunocytochemistry revealed OPN4 expression at the base of the rhabdomeres, mainly at the rhabdomeral stalk. Measurements of the early receptor current, a linear manifestation of photopigment activation, indicated large expression of OPN4 in the plasma membrane. Comparing the early receptor current amplitude and action spectra between WT and the Opn4-expressing Drosophila further indicated that large quantities of a blue absorbing photopigment were expressed, having a dark stable blue intermediate state. Strikingly, the light-induced current of the Opn4-expressing fly photoreceptors was ∼40-fold faster than that of ipRGC. Furthermore, an intense white flash induced a small amplitude prolonged dark current composed of discrete unitary currents similar to the Drosophila single photon responses. The induction of prolonged dark currents by intense blue light could be suppressed by a following intense green light, suggesting induction and suppression of prolonged depolarizing afterpotential. This is the first demonstration of heterologous functional expression of mammalian OPN4 in the genetically emendable Drosophila photoreceptors. Moreover, the fast OPN4-activated ionic current of Drosophila photoreceptors relative to that of mouse ipRGC, indicates that the slow light response of ipRGC does not arise from an intrinsic property of melanopsin.

Photoreception and vision in the ultraviolet

Ultraviolet (UV) light occupies the spectral range of wavelengths slightly shorter than those visible to humans. Because of its shorter wavelength, it is more energetic (and potentially more photodamaging) than ‘visible light’, and it is scattered more efficiently in air and water. Until 1990, only a few animals were recognized as being sensitive to UV light, but we now know that a great diversity, possibly even the majority, of animal species can visually detect and respond to it. Here, we discuss the history of research on biological UV photosensitivity and review current major research trends in this field. Some animals use their UV photoreceptors to control simple, innate behaviors, but most incorporate their UV receptors into their general sense of vision. They not only detect UV light but recognize it as a separate color in light fields, on natural objects or living organisms, or in signals displayed by conspecifics. UV visual pigments are based on opsins, the same family of proteins that are used to detect light in conventional photoreceptors. Despite some interesting exceptions, most animal species have a single photoreceptor class devoted to the UV. The roles of UV in vision are manifold, from guiding navigation and orientation behavior, to detecting food and potential predators, to supporting high-level tasks such as mate assessment and intraspecific communication. Our current understanding of UV vision is restricted almost entirely to two phyla: arthropods and chordates (specifically, vertebrates), so there is much comparative work to be done.

Advances in understanding the molecular basis of the first steps in color vision

Serving as one of our primary environmental inputs, vision is the most sophisticated sensory system in humans. Here, we present recent findings derived from energetics, genetics and physiology that provide a more advanced understanding of color perception in mammals. Energetics of cis-trans isomerization of 11-cis-retinal accounts for color perception in the narrow region of the electromagnetic spectrum and how human eyes can absorb light in the near infrared (IR) range. Structural homology models of visual pigments reveal complex interactions of the protein moieties with the light sensitive chromophore 11-cis-retinal and that certain color blinding mutations impair secondary structural elements of these G protein-coupled receptors (GPCRs). Finally, we identify unsolved critical aspects of color tuning that require future investigation.

Visual circuits in flies: beginning to see the whole picture

Sensory signals are processed in the brain by dedicated neuronal circuits to form perceptions used to guide behavior. Drosophila, with its compact brain, amenability to genetic manipulations and sophisticated behaviors has emerged as a powerful model for investigating the neuronal circuits responsible for sensory perception. Vision in particular has been examined in detail. Light is detected in the eye by photoreceptors, specialized neurons containing light sensing Rhodopsin proteins. These photoreceptor signals are relayed to the optic lobes where they are processed to gain perceptions about different properties of the visual scene. In this review we describe recent advances in the characterization of neuronal circuits underlying four visual modalities in the fly brain: motion vision, phototaxis, color and polarized light vision.

Visual ecology of flies with particular reference to colour vision and colour preferences

The visual ecology of flies is outstanding among insects due to a combination of specific attributes. Flies' compound eyes possess an open rhabdom and thus separate rhabdomeres in each ommatidium assigned to two visual pathways. The highly sensitive, monovariant neural superposition system is based on the excitation of the peripheral rhabdomeres of the retinula cells R1-6 and controls optomotor reactions. The two forms of central rhabdomeres of R7/8 retinula cells in each ommatidium build up a system with four photoreceptors sensitive in different wavelength ranges and thought to account for colour vision. Evidence from wavelength discrimination tests suggests that all colour stimuli are assigned to one of just four colour categories, but cooperation of the two pathways is also evident. Flies use colour cues for various behavioural reactions such as flower visitation, proboscis extension, host finding, and egg deposition. Direct evidence for colour vision, the ability to discriminate colours according to spectral shape but independent of intensity, has been demonstrated for few fly species only. Indirect evidence for colour vision provided from electrophysiological recordings of the spectral sensitivity of photoreceptors and opsin genes indicates similar requisites in various flies; the flies' responses to coloured targets, however, are much more diverse.

Vision in Drosophila: seeing the world through a model's eyes

The fruit fly, Drosophila melanogaster, has been used for decades as a genetic model for unraveling mechanisms of development and behavior. In order to efficiently assign gene functions to cellular and behavioral processes, early measures were often necessarily simple. Much of what is known of developmental pathways was based on disrupting highly regular structures, such as patterns of cells in the eye. Similarly, reliable visual behaviors such as phototaxis and motion responses provided a solid foundation for dissecting vision. Researchers have recently begun to examine how this model organism responds to more complex or naturalistic stimuli by designing novel paradigms that more closely mimic visual behavior in the wild. Alongside these advances, the development of brain-recording strategies allied with novel genetic tools has brought about a new era of Drosophila vision research where neuronal activity can be related to behavior in the natural world.

Multiple spectral inputs improve motion discrimination in the Drosophila visual system

Color and motion information are thought to be channeled through separate neural pathways, but it remains unclear whether and how these pathways interact to improve motion perception. In insects, such as Drosophila, it has long been believed that motion information is fed exclusively by one spectral class of photoreceptor, so-called R1 to R6 cells; whereas R7 and R8 photoreceptors, which exist in multiple spectral classes, subserve color vision. Here, we report that R7 and R8 also contribute to the motion pathway. By using electrophysiological, optical, and behavioral assays, we found that R7/R8 information converge with and shape the motion pathway output, explaining flies' broadly tuned optomotor behavior by its composite responses. Our results demonstrate that inputs from photoreceptors of different spectral sensitivities improve motion discrimination, increasing robustness of perception.

Evolution of eye morphology and rhodopsin expression in the Drosophila melanogaster species subgroup

A striking diversity of compound eye size and shape has evolved among insects. The number of ommatidia and their size are major determinants of the visual sensitivity and acuity of the compound eye. Each ommatidium is composed of eight photoreceptor cells that facilitate the discrimination of different colours via the expression of various light sensitive Rhodopsin proteins. It follows that variation in eye size, shape, and opsin composition is likely to directly influence vision. We analyzed variation in these three traits in D. melanogaster, D. simulans and D. mauritiana. We show that D. mauritiana generally has larger eyes than its sibling species, which is due to a combination of larger ommatidia and more ommatidia. In addition, intra- and inter-specific differences in eye size among D. simulans and D. melanogaster strains are mainly caused by variation in ommatidia number. By applying a geometric morphometrics approach to assess whether the formation of larger eyes influences other parts of the head capsule, we found that an increase in eye size is associated with a reduction in the adjacent face cuticle. Our shape analysis also demonstrates that D. mauritiana eyes are specifically enlarged in the dorsal region. Intriguingly, this dorsal enlargement is associated with enhanced expression of rhodopsin 3 in D. mauritiana. In summary, our data suggests that the morphology and functional properties of the compound eyes vary considerably within and among these closely related Drosophila species and may be part of coordinated morphological changes affecting the head capsule.

The history of the prolonged depolarizing afterpotential (PDA) and its role in genetic dissection of Drosophila phototransduction

In invertebrate photoreceptors, the photopigment exhibits a long-lived and physiologically active photoproduct, called metarhodopsin (M). The long life of invertebrate M implies that under physiological conditions, M and the original pigment state rhodopsin, R, are in photoequilibrium. In many invertebrates, the absorption spectra of R and M states are different, allowing large photopigment conversion between R and M states. These net pigment molecules conversions between R and M are the basis of the prolonged depolarizing afterpotential (PDA) phenomenology, which is the main subject of this review. A large net conversion of R to M disrupts phototransduction termination at the photopigment level, which in turn results in sustained excitation long after the light is turned off. Throughout this period, the photoreceptors are partially desensitized and are insensitive (or less sensitive) to subsequent test lights. In Drosophila, the PDA tests the maximal capacity of the photoreceptor cell to maintain excitation for an extended period and is strictly dependent on the presence of high concentrations of rhodopsin and the transient receptor potential (TRP) channels. Therefore, it detects even minor defects in rhodopsin or TRP biogenesis and easily scores deficient replenishment of phototransduction components, which results in temporary desensitization of the phototransduction process. Indeed, the introduction and use of PDA to screen for phototransduction-defective Drosophila mutants by Pak and colleagues yielded a plethora of new and most interesting visual mutants. Remarkably, to this day, the PDA mutants that Pak and his colleagues isolated are the main source of mutants for analysis of the Drosophila visual system.

The retinal mosaics of opsin expression in invertebrates and vertebrates

Color vision is found in many invertebrate and vertebrate species. It is the ability to discriminate objects based on the wavelength of emitted light independent of intensity. As it requires the comparison of at least two photoreceptor types with different spectral sensitivities, this process is often mediated by a mosaic made of several photoreceptor types. In this review, we summarize the current knowledge about the formation of retinal mosaics and the regulation of photopigment (opsin) expression in the fly, mouse, and human retina. Despite distinct evolutionary origins, as well as major differences in morphology and phototransduction machineries, there are significant similarities in the stepwise cell-fate decisions that lead from progenitor cells to terminally differentiated photoreceptors that express a particular opsin. Common themes include (i) the use of binary transcriptional switches that distinguish classes of photoreceptors, (ii) the use of gradients of signaling molecules for regional specializations, (iii) stochastic choices that pattern the retina, and (iv) the use of permissive factors with multiple roles in different photoreceptor types.

Stochastic de-repression of Rhodopsins in single photoreceptors of the fly retina

The photoreceptors of the Drosophila compound eye are a classical model for studying cell fate specification. Photoreceptors (PRs) are organized in bundles of eight cells with two major types – inner PRs involved in color vision and outer PRs involved in motion detection. In wild type flies, most PRs express a single type of Rhodopsin (Rh): inner PRs express either Rh3, Rh4, Rh5 or Rh6 and outer PRs express Rh1. In outer PRs, the K₅₀ homeodomain protein Dve is a key repressor that acts to ensure exclusive Rh expression. Loss of Dve results in de-repression of Rhodopsins in outer PRs, and leads to a wide distribution of expression levels. To quantify these effects, we introduce an automated image analysis method to measure Rhodopsin levels at the single cell level in 3D confocal stacks. Our sensitive methodology reveals cell-specific differences in Rhodopsin distributions among the outer PRs, observed over a developmental time course. We show that Rhodopsin distributions are consistent with a two-state model of gene expression, in which cells can be in either high or basal states of Rhodopsin production. Our model identifies a significant role of post-transcriptional regulation in establishing the two distinct states. The timescale for interconversion between basal and high states is shown to be on the order of days. Our results indicate that even in the absence of Dve, the Rhodopsin regulatory network can maintain highly stable states. We propose that the role of Dve in outer PRs is to buffer against rare fluctuations in this network.

Complex networks of genetic interactions govern the development of multicellular organisms. One of the best-characterized networks governs the development of the fruit-fly retina, a highly organized, three-dimensional organ composed of a hexagonal grid of eight types of photoreceptor neurons. Each photoreceptor responds to a particular wavelength of light depending on the Rhodopsin protein it expresses. We present novel computational methods to quantify cell-specific Rhodopsin levels from confocal microscopy images. We apply these methods to study the effect of the loss of a key repressor that ensures each photoreceptor expresses only one Rhodopsin. We show that this perturbation has cell-specific effects. Our measurement of the cell-type specific Rhodopsin distributions reveals differences between photoreceptor cells, which could not otherwise be detected. Using mathematical models of gene expression, we attribute this variability to stochastic events that activate Rhodopsin production.

Edge detection depends on achromatic channel in Drosophila melanogaster

Edges represent important information in object recognition, and thus edge detection is crucial for animal survival. Various types of edges result from visual contrast, such as luminance contrast and color contrast. Thus far, the molecular and neural mechanisms underlying edge detection and the relationship between different edge information-processing pathways have largely been undemonstrated. In the present study, using a color-light-emitting diode (LED)-based Buridan's paradigm, we demonstrated that a blue/green demarcation is able to generate edge-orientation behavior in the adult fly. There is a blue/green intensity ratio, the so-called point of equal-luminance (POE), at which wild-type flies did not show obvious orientation behavior towards edges. This suggests that orientation behavior towards edges is dependent on luminance contrast in Drosophila. The results of mutants ninaE17 and sevLY3;rh52;rh61 demonstrated that achromatic R1-R6 photoreceptor cells, but not chromatic R7/R8 photoreceptor cells, were necessary for orientation behavior towards edges. Moreover, ectopic expression of Rh4, Rh5 or Rh6 could efficiently restore the edge-orientation defect in the ninaE17 mutant. Altogether, our results show that R1-R6 photoreceptor cells are both necessary and sufficient for orientation behavior towards edges in Drosophila.

On visual pigment templates and the spectral shape of invertebrate rhodopsins and metarhodopsins

The absorbance spectra of visual pigments can be approximated with mathematical expressions using as single parameter the absorbance peak wavelength. A comparison of the formulae of Stavenga et al. in Vision Res 33:1011–1017 (1993) and Govardovskii et al. in Vis Neurosci 17:509–528 (2000) applied to a number of invertebrate rhodopsins reveals that both templates well describe the normalized α-band of rhodopsins with peak wavelength > 400 nm; the template spectra are virtually indistinguishable in an absorbance range of about three log units. The template formulae of Govardovskii et al. in Vis Neurosci 17:509–528 (2000) describe the rhodopsin spectra better for absorbances below 10⁻³. The template predicted spectra deviate in the ultraviolet wavelength range from each other as well as from measured spectra, preventing a definite conclusion about the spectral shape in the wavelength range <400 nm. The metarhodopsin spectra of blowfly and fruitfly R1-6 photoreceptors derived from measured data appear to be virtually identical. The established templates describe the spectral shape of fly metarhodopsin reasonably well. However, the best fitting template spectrum slightly deviates from the experimental spectra near the peak and in the long-wavelength tail. Improved formulae for fitting the fly metarhodopsin spectra are proposed.

Contribution of photoreceptor subtypes to spectral wavelength preference in Drosophila

The visual systems of most species contain photoreceptors with distinct spectral sensitivities that allow animals to distinguish lights by their spectral composition. In Drosophila, photoreceptors R1-R6 have the same spectral sensitivity throughout the eye and are responsible for motion detection. In contrast, photoreceptors R7 and R8 exhibit heterogeneity and are important for color vision. We investigated how photoreceptor types contribute to the attractiveness of light by blocking the function of certain subsets and by measuring differential phototaxis between spectrally different lights. In a "UV vs. blue" choice, flies with only R1-R6, as well as flies with only R7/R8 photoreceptors, preferred blue, suggesting a nonadditive interaction between the two major subsystems. Flies defective for UV-sensitive R7 function preferred blue, whereas flies defective for either type of R8 (blue- or green-sensitive) preferred UV. In a "blue vs. green" choice, flies defective for R8 (blue) preferred green, whereas those defective for R8 (green) preferred blue. Involvement of all photoreceptors [R1-R6, R7, R8 (blue), R8 (green)] distinguishes phototaxis from motion detection that is mediated exclusively by R1-R6.

Peripheral visual circuits functionally segregate motion and phototaxis behaviors in the fly

Like the mammalian visual cortex, the fly visual system is organized into retinotopic columns. A widely accepted biophysical model for computing visual motion, the elementary motion detector proposed nearly 50 years ago posits a temporal correlation of spatially separated visual inputs implemented across neighboring retinotopic visual columns. Whereas the inputs are defined, the neural substrate for motion computation remains enigmatic. Indeed, it is not known where in the visual processing hierarchy the computation occurs. Here, we combine genetic manipulations with a novel high-throughput dynamic behavioral analysis system to dissect visual circuits required for directional optomotor responses. An enhancer trap screen of synapse-inactivated neural circuits revealed one particularly striking phenotype, which is completely insensitive to motion yet displays fully intact fast phototaxis, indicating that these animals are generally capable of seeing and walking but are unable to respond to motion stimuli. The enhancer circuit is localized within the first optic relay and strongly labels the only columnar interneuron known to interact with neighboring columns both in the lamina and medulla, spatial synaptic interactions that correspond with the two dominant axes of elementary motion detectors on the retinal lattice.

Xanthorhodopsin: a bacteriorhodopsin-like proton pump with a carotenoid antenna

Xanthorhodopsin is a light-driven proton pump like bacteriorhodopsin, but made more effective for collecting light by its second chromophore, salinixanthin, a carotenoid. Action spectra for transport and fluorescence of the retinal upon excitation of the carotenoid indicate that the carotenoid functions as an antenna to the retinal. The calculated center-to-center distance and angle of the transition moments of the two chromophores are 11 A and 56 degrees , respectively. As expected from their proximity, the carotenoid and the retinal closely interact: tight binding of the carotenoid, as indicated by its sharpened vibration bands and intense induced circular dichroism in the visible, is removed by hydrolysis of the retinal Schiff base, and restored upon reconstitution with retinal. This antenna system, simpler than photosynthetic complexes, is well-suited to study features of excited-state energy migration.

Photochemistry of retinal chromophore in mouse melanopsin

In mammals, melanopsin is exclusively expressed in intrinsically photosensitive retinal ganglion cells (ipRGCs), which play an important role in circadian photoentrainment and other nonimage-forming functions. These ipRGCs reside in the inner retina, far removed from the pigment epithelium, which synthesizes the 11-cis retinal chromophore used by rod and cone photoreceptors to regenerate opsin for light detection. There has been considerable interest in the identification of the melanopsin chromophore and in understanding the process of photopigment regeneration in photoreceptors that are not in proximity to the classical visual cycle. We have devised an immuno-magnetic purification protocol that allows melanopsin-expressing retinal ganglion cells to be isolated and collected from multiple mouse retinas. Using this technique, we have demonstrated that native melanopsin in vivo exclusively binds 11-cis retinal in the dark and that illumination causes isomerization to the all-trans isoform. Furthermore, spectral analysis of the melanopsin photoproduct shows the formation of a protonated metarhodopsin with a maximum absorbance between 520 and 540 nm. These results indicate that even if melanopsin functions as a bistable photopigment with photo-regenerative activity native melanopsin must also use some other light-independent retinoid regeneration mechanism to return to the dark state, where all of the retinal is observed to be in the 11-cis form.

Excitation energy-transfer and the relative orientation of retinal and carotenoid in xanthorhodopsin

The cell membrane of Salinibacter ruber contains xanthorhodopsin, a light-driven transmembrane proton pump with two chromophores: a retinal and the carotenoid, salinixanthin. Action spectra for transport had indicated that light absorbed by either is utilized for function. If the carotenoid is an antenna in this protein, its excited state energy has to be transferred to the retinal and should be detected in the retinal fluorescence. From fluorescence studies, we show that energy transfer occurs from the excited singlet S(2) state of salinixanthin to the S(1) state of the retinal. Comparison of the absorption spectrum with the excitation spectrum for retinal emission yields 45 +/- 5% efficiency for the energy transfer. Such high efficiency would require close proximity and favorable geometry for the two polyene chains, but from the heptahelical crystallographic structure of the homologous retinal protein, bacteriorhodopsin, it is not clear where the carotenoid can be located near the retinal. The fluorescence excitation anisotropy spectrum reveals that the angle between their transition dipole moments is 56 +/- 3 degrees . The protein accommodates the carotenoid as a second chromophore in a distinct binding site to harvest light with both extended wavelength and polarization ranges. The results establish xanthorhodopsin as the simplest biological excited-state donor-acceptor system for collecting light.

Motion vision is independent of color in Drosophila

Whether motion vision uses color contrast is a controversial issue that has been investigated in several species, from insects to humans. We used Drosophila to answer this question, monitoring the optomotor response to moving color stimuli in WT and genetic variants. In the fly eye, a motion channel (outer photoreceptors R1-R6) and a color channel (inner photoreceptors R7 and R8) have been distinguished. With moving bars of alternating colors and high color contrast, a brightness ratio of the two colors can be found, at which the optomotor response is largely missing (point of equiluminance). Under these conditions, mutant flies lacking functional rhodopsin in R1-R6 cells do not respond at all. Furthermore, genetically eliminating the function of photoreceptors R7 and R8 neither alters the strength of the optomotor response nor shifts the point of equiluminance. We conclude that the color channel (R7/R8) does not contribute to motion detection as monitored by the optomotor response.

Senseless functions as a molecular switch for color photoreceptor differentiation in Drosophila

A major question in development is how different specialized cell types arise from a common progenitor. In the adult Drosophila compound eye, color discrimination is achieved by UV-, blue- and green-sensitive photoreceptors (PRs). These different PR subsets arise from neuronal precursors called R7 and R8 cells. Recent studies have demonstrated that R7-based UV-sensitive PRs require the repression of R8-based blue/green-sensitive PR characteristics to properly develop. This repression is mediated by the transcription factor Prospero (Pros). Here, we report that Senseless (Sens), a Drosophila ortholog of the vertebrate Gfi1 transcription factor, plays an opposing role to Pros by both negatively regulating R7-based features and positively enforcing R8-based features during terminal differentiation. In addition, we demonstrate that Pros and Sens function together with the transcription factor Orthodenticle (Otd) to oppositely regulate R7 and R8 PR Rhodopsin gene expression in vitro. These data show that sens, previously shown to be essential for neuronal specification, also controls differentiation of specific neuronal subtypes in the retina. Interestingly, Pros has recently been shown to function as a tumor suppressor, whereas Gfi1 is a well-characterized oncogene. Thus, we propose that sens/pros antagonism is important for regulating many biological processes.

Chromatic cues to trap the oriental fruit fly, Bactrocera dorsalis

Various colors have been used as visual cues to trap insect pests. For example, yellow traps for monitoring and control of the oriental fruit fly (Bactrocera dorsalis) have been in use for a very long time. However, the chromatic cue of using color traps has never been meticulously investigated. In this study, the spectral sensitivities of the photoreceptors in the compound eyes of B. dorsalis were measured intracellularly, and the theory of receptor quantum catch was applied to study the chromatic cue of fly attracting. Responses to five wavelength categories with peak wavelengths of 370, 380, 490, and 510 nm, and one with dual peaks at 350 and 490 nm were recorded. Based on spectral sensitivities, six colored papers were chosen to test the color preference of the fly, and an additional UV preference test was done to confirm the effect of the UV stimuli. It was concluded that UV and green stimuli (spectra: 300-380 nm and 500-570 nm) would enhance the attractiveness of a colored paper to the oriental fruit fly, and blue stimuli (380-500 nm) would diminish the attractiveness.

Direction-selective adaptation in fly visual motion-sensitive neurons is generated by an intrinsic conductance-based mechanism

Motion-sensitive neurons in the blowfly brain present an ideal model system to study the cellular mechanisms and functional significance of adaptation to visual motion. Various adaptation processes have been described, but it is still largely unknown which of these processes are generated in the motion-sensitive neurons themselves and which originate at more peripheral processing stages. By input resistance measurements I demonstrate that direction-selective adaptation is generated by an activity-dependent conductance increase in the motion-sensitive neurons. Based on correlations between dendritic Ca(2+) accumulation and slow hyperpolarizing after-potentials following excitatory stimulation, a regulation of direction-selective adaptation by Ca(2+) has previously been suggested. In the present study, however, adaptation phenomena are not evoked when the cytosolic Ca(2+) concentration is elevated by ultraviolet photolysis of caged Ca(2+) in single neurons rather than by motion stimulation. This result renders it unlikely, that adaptation in fly motion-sensitive neurons is regulated by bulk cytosolic Ca(2+).

Chlorophyll derivatives as visual pigments for super vision in the red

The primary event in vision is light-initiated activation of visual pigments. All visual pigments consist of the protein opsin bound to 11-cis-retinal and are responsible for initiating the transformation of light into an electrical signal. In a mouse model, we show that derivatives of chlorophyll can act as visual pigments initiating the transformation of light into an electrical signal and thus change the primary event in vision to initial activation of a chlorophyll derivative. Electroretinographic b-wave amplitudes recorded in response to red and blue light were two-fold greater in mice administered chlorin e(6), which accumulated in photoreceptor outer segments.

The Drosophila ninaG Oxidoreductase Acts in Visual Pigment Chromophore Production

The Drosophila ninaG mutant is characterized by low levels of Rh1 rhodopsin, because of the inability to transport this rhodopsin from the endoplasmic reticulum to the rhabdomere. ninaG mutants do not affect the biogenesis of the minor opsins Rh4 and Rh6. A genetic analysis placed the ninaG gene within the 86E4-86E6 chromosomal region. A sequence analysis of the 15 open reading frames within this region from the ninaG(P330) mutant allele identified a stop codon in the CG6728 gene. Germ-line transformation of the CG6728 genomic region rescued the ninaG mutant phenotypes, confirming that CG6728 corresponds to the ninaG gene. The NinaG protein belongs to the glucose-methanol-choline oxidoreductase family of flavin adenine dinucleotide-binding enzymes catalyzing hydroxylation and oxidation of a variety of small organic molecules. High performance liquid chromatography analysis of retinoids was used to gain insight into the in vivo role of the NinaG oxidoreductase. The results show that when Rh1 is expressed as the major rhodopsin, ninaG flies fail to accumulate 3-hydroxyretinal. Further, in transgenic flies expressing Rh4 as the major rhodopsin, 3-hydroxyretinal is the major retinoid in ninaG+, but a different retinoid profile is observed in ninaG(P330). These results indicate that the ninaG oxidoreductase acts in the biochemical pathway responsible for conversion of retinal to the rhodopsin chromophore, 3-hydroxyretinal.

Light-dependent subcellular translocation of Gqα in Drosophila photoreceptors is facilitated by the photoreceptor-specific myosin III NINAC

We examine the light-dependent subcellular translocation of the visual Gqα protein between the signaling compartment, the rhabdomere and the cell body in Drosophila photoreceptors. We characterize the translocation of Gqα and provide the first evidence implicating the involvement of the photoreceptor-specific myosin III NINAC in Gqα transport. Translocation of Gqα from the rhabdomere to the cell body is rapid, taking less than 5 minutes. Higher light intensities increased the quantity of Gqα translocated out of the rhabdomeres from 20% to 75%, consistent with a mechanism for light adaptation. We demonstrate that translocation of Gqα requires rhodopsin, but none of the known downstream phototransduction components, suggesting that the signaling pathway triggering translocation occurs upstream of Gqα. Finally, we show that ninaC mutants display a significantly reduced rate of Gqα transport from the cell body to the rhabdomere, suggesting that NINAC might function as a light-dependent plus-end motor involved in the transport of Gqα.

Visual acuity of fly photoreceptors in natural conditions--dependence on UV sensitizing pigment and light-controlling pupil

The effect of the UV-absorbing sensitizing pigment of fly photoreceptors on absolute, spectral and angular sensitivity was investigated with a wave-optics model for the facet lens-rhabdomere system. When sky light was used as a UV-rich light source, one sensitizing pigment molecule per rhodopsin increased the photoreceptor absorption by 14-18% with respect to pure rhodopsin, whilst two sensitizing pigment molecules per rhodopsin increased the absorption by 20-27%. Upon light adaptation, when the pupil mechanism is activated, photoreceptor absorption decreases; in the housefly, Musca, by up to 6-fold. The fully light-adapted pupil diminishes the photoreceptor's acceptance angle by a factor of approximately 0.6 due to selective absorption of higher order waveguide modes. Spatial acuity of dark-adapted photoreceptors is more or less constant throughout the visual wavelength range, including the UV, because the waveguide optics of the rhabdomere compromise acuity least at wavelengths most limited by diffraction of the facet lens. Diffraction is not the general limiting factor causative for UV sensitivity of insect eyes. Visual acuity is governed by diffraction only with a fully light-adapted pupil, which absorbs higher waveguide modes. Closure of the blue-absorbing pupil causes a UV-peaking spectral sensitivity of fly photoreceptors. The sensitizing pigment does not play an appreciable role in modifying spatial acuity, neither in the dark- nor the light-adapted state, due to the dominant contribution of green light in natural light sources.

Photic input pathways that mediate theDrosophila larval response to light and circadian rhythmicity are developmentally related but functionally distinct

The Drosophila melanogaster larval photosensory organ that mediates the response to light consists of bilaterally symmetrical clusters of 12 photoreceptors. These are distinguished on the basis of expression of the rhodopsins Rh5 and Rh6. The Rh6-expressing cells correspond to the Hofbauer-Buchner (H-B) eyelet found later in the posterior margin of the adult compound eye and recently shown to function as an input pathway in the entrainment of circadian rhythmicity in adult Drosophila. In addition, the axons of the larval photoreceptors are found in intimate association with a subset of the main circadian pacemaker neurons located in the developing accessory medulla, the small ventral lateral neurons (LNv). The observed spatial overlap between components of the circadian circuitry, input pathway, and pacemaker neurons-and the larval visual organ-suggest a functional relationship between these two photosensory input pathways. In this study we determined the requirement of specific rhodopsin-expressing photoreceptors including the presumptive H-B eyelet and pacemaker neurons in the larval locomotory response to visual stimuli. Our results demonstrate that two of the most important components of the neuronal circuitry underlying circadian rhythmicity in Drosophila, namely, the extraretinal H-B cluster and the circadian pacemakers, while in intimate association with the larval visual system are not required for the larval motor response to light.

Molecular basis for ultraviolet vision in invertebrates

Invertebrates are sensitive to a broad spectrum of light that ranges from UV to red. Color sensitivity in the UV plays an important role in foraging, navigation, and mate selection in both flying and terrestrial invertebrate animals. Here, we show that a single amino acid polymorphism is responsible for invertebrate UV vision. This residue (UV: lysine vs blue:asparagine or glutamate) corresponds to amino acid position glycine 90 (G90) in bovine rhodopsin, a site affected in autosomal dominant human congenital night blindness. Introduction of the positively charged lysine in invertebrates is likely to deprotonate the Schiff base chromophore and produce an UV visual pigment. This same position is responsible for regulating UV versus blue sensitivity in several bird species, suggesting that UV vision has arisen independently in invertebrate and vertebrate lineages by a similar molecular mechanism.

Heterologous expression of limulus rhodopsin

Invertebrates such as Drosophila or Limulus assemble their visual pigment into the specialized rhabdomeric membranes of photoreceptors where phototransduction occurs. We have investigated the biosynthesis of rhodopsin from the Limulus lateral eye with three cell culture expression systems: mammalian COS1 cells, insect Sf9 cells, and amphibian Xenopus oocytes. We extracted and affinity-purified epitope-tagged Limulus rhodopsin expressed from a cDNA or cRNA from these systems. We found that all three culture systems could efficiently synthesize the opsin polypeptide in quantities comparable with that found for bovine opsin. However, none of the systems expressed a protein that stably bound 11-cis-retinal. The protein expressed in COS1 and Sf9 cells appeared to be misfolded, improperly localized, and proteolytically degraded. Similarly, Xenopus oocytes injected with Limulus opsin cRNA did not evoke light-sensitive currents after incubation with 11-cis-retinal. However, injecting Xenopus oocytes with mRNA from Limulus lateral eyes yielded light-dependent conductance changes after incubation with 11-cis-retinal. Also, expressing Limulus opsin cDNA in the R1-R6 photoreceptors of transgenic Drosophila yielded a visual pigment that bound retinal, had normal spectral properties, and coupled to the endogenous phototransduction cascade. These results indicate that Limulus opsin may require one or more photoreceptor-specific proteins for correct folding and/or chromophore binding. This may be a general property of invertebrate opsins and may underlie some of the functional differences between invertebrate and vertebrate visual pigments.

Otd/Crx, a dual regulator for the specification of ommatidia subtypes in the Drosophila retina

Comparison between the inputs of photoreceptors with different spectral sensitivities is required for color vision. In Drosophila, this is achieved in each ommatidium by the inner photoreceptors R7 and R8. Two classes of ommatidia are distributed stochastically in the retina: 30% contain UV-Rh3 in R7 and blue-Rh5 in R8, while the remaining 70% contain UV-Rh4 in R7 and green-Rh6 in R8. We show here that the distinction between the rhodopsins expressed in the two classes of ommatidia depends on a series of highly conserved homeodomain binding sites present in the rhodopsin promoters. The homeoprotein Orthodenticle acts through these sites to activate rh3 and rh5 in their specific ommatidial subclass and through the same sites to prevent rh6 expression in outer photoreceptors. Therefore, Otd is a key player in the terminal differentiation of subtypes of photoreceptors by regulating rhodopsin expression, a function reminiscent of the role of one of its mammalian homologs, Crx, in eye development.

A new visualization approach for identifying mutations that affect differentiation and organization of the Drosophila ommatidia

The Drosophila eye is widely used as a model system to study neuronal differentiation, survival and axon projection. Photoreceptor differentiation starts with the specification of a founder cell R8, which sequentially recruits other photoreceptor neurons to the ommatidium. The eight photoreceptors that compose each ommatidium exist in two chiral forms organized along two axes of symmetry and this pattern represents a paradigm to study tissue polarity. We have developed a method of fluoroscopy to visualize the different types of photoreceptors and the organization of the ommatidia in living animals. This allowed us to perform an F(1) genetic screen to isolate mutants affecting photoreceptor differentiation, survival or planar polarity. We illustrate the power of this detection system using known genetic backgrounds and new mutations that affect ommatidial differentiation, morphology or chirality.

Characterization of the primary photointermediates of Drosophila rhodopsin

Invertebrate opsins are unique among the visual pigments because the light-activated conformation, metarhodopsin, is stable following exposure to light in vivo. Recovery of the light-activated pigment to the dark conformation (or resting state) occurs either thermally or photochemically. There is no evidence to suggest that the chromophore becomes detached from the protein during any stage in the formation or recovery processes. Biochemical and structural studies of invertebrate opsins have been limited by the inability to express and purify rhodopsins for structure-function studies. In this study, we used Drosophila to produce an epitope-tagged opsin, Rh1-1D4, in quantities suitable for spectroscopic and photochemical characterization. When expressed in Drosophila, Rh1-1D4 is localized to the rhabdomere membranes, has the same spectral properties in vivo as wild-type Rh1, and activates the phototransduction cascade in a normal manner. Purified Rh1-1D4 visual pigment has an absorption maximum of the dark-adapted state of 474 nm, while the metarhodopsin absorption maximum is 572 nm. However, the metarhodopsin state is not stable as purified in dodecyl maltoside but decays with kinetics that require a double-exponential fit having lifetimes of 280 and 2700 s. We investigated the primary properties of the pigment at low temperature. At 70 K, the pigment undergoes a temperature-induced red shift to 486 nm. Upon illumination with 435 nm light, a photostationary state mixture is formed consisting of bathorhodopsin (lambda(max) = 545 nm) and isorhodopsin (lambda(max) = 462 nm). We also compared the spectroscopic and photochemical properties of this pigment with other vertebrate pigments. We conclude that the binding site of Drosophila rhodopsin is similar to that of bovine rhodopsin and is characterized by a protonated Schiff base chromophore stabilized via a single negatively charged counterion.

Key issues in the photochemistry and signalling-state formation of photosensor proteins

Four families of photosensors (i.e., rhodopsins, phytochromes, xanthopsins and cryptochromes) exist, which vary widely in the degree to which we understand the molecular basis of their activity. Some of their members are ideal model systems for studying the structure-function relation of proteins, and the role of dynamics therein. The photochemistry of photosensor activation is based upon the cis <--> trans isomerization of the chromophore. This configurational transition leads to the formation of a signalling state of sufficient stability to communicate the presence of photons to a downstream signal-transduction partner. In the xanthopsins it has been demonstrated that the exact nature of this signalling state is strongly dependent on the mesoscopic context of the sensor protein. The cryptochromes appear to challenge the photoisomerization rule.

Blue- and green-absorbing visual pigments of Drosophila: ectopic expression and physiological characterization of the R8 photoreceptor cell-specific Rh5 and Rh6 rhodopsins

Color discrimination requires the input of different photoreceptor cells that are sensitive to different wavelengths of light. The Drosophila visual system contains multiple classes of photoreceptor cells that differ in anatomical location, synaptic connections, and spectral sensitivity. The Rh5 and Rh6 opsins are expressed in nonoverlapping sets of R8 cells and are the only Drosophila visual pigments that remain uncharacterized. In this study, we ectopically expressed Rh5 and Rh6 in the major class of photoreceptor cells (R1-R6) and show them to be biologically active in their new environment. The expression of either Rh5 or Rh6 in "blind" ninaE(17) mutant flies, which lack the gene encoding the visual pigment of the R1-R6 cells, fully rescues the light response. Electrophysiological analysis showed that the maximal spectral sensitivity of the R1-R6 cells is shifted to 437 or 508 nm when Rh5 or Rh6, respectively, is expressed in these cells. These spectral sensitivities are in excellent agreement with intracellular recordings of the R8p and R8y cells measured in Calliphora and Musca. Spectrophotometric analyses of Rh5 and Rh6 in vivo by microspectrophotometry, and of detergent-extracted pigments in vitro, showed that Rh5 is reversibly photoconverted to a stable metarhodopsin (lambda(max) = 494 nm), whereas Rh6 appears to be photoconverted to a metarhodopsin (lambda(max) = 468 nm) that is less thermally stable. Phylogenetically, Rh5 belongs to a group of short-wavelength-absorbing invertebrate visual pigments, whereas Rh6 is related to a group of long-wavelength-absorbing pigments and is the first member of this class to be functionally characterized.